Bell Ringers Map

Here is a snapshot of a map (updated 2/18/2010) that includes most of the Bell Ringers who appear on 40 or 75 m.  Click on the image to magnify or to download. In this view I've left off the details of cities and roads to provide a less cluttered view of were our members are located.

The placemarks are defined in a KML file that can display in either Google Earth or Google Maps (for viewing in a web browser). Let me know if you'd like to see a different view or if you want to make some yourself using a copy of the KML file.

Radios with Micromachined Resonators

An article in the December 2009 issue of IEEE Spectrum describes how precision micromachined mechanical components, especially resonators, can provide performance superior to that of electronic components for key functions in wireless handsets (e.g. cell phones).  Two valuable characteristics of such components are:

• zero or very low consumption of battery power
• very high Q in resonant circuits (e.g. > 10,000 at 1.5 GHz)
  

 Dr. Nguyen argues that electronic solutions, including software defined radio designs, are not likely to be practical for wireless handsets due to their requirements on battery power. He points to the presence, in today's cell phone designs, of certain electro-mechanical components:

• thin-film bulk acoustic resonators (FBARs), 
• surface acoustic-wave (SAW) resonators, and 
• quartz crystals.

He predicts that future designs will incorporate MEMS components to overcome the challenges of purely electronic approaches to radio front ends. He reports that handset makers and DARPA are funding work in this area. For additional background on this work, with lots of nice illustrations and article links, visit Dr. Nguyen's web site which includes this link to a research summary.

Saturday Net on 7230 kHz

The signal reports among participants in the Saturday, 26 Dec 2009 net confirmed the continuing lack of propagation at distances of less than about 250 miles. Without a map in front of me that shows the locations of participants, it is hard to visualize the relative distances and directions among stations. Below is a map view showing the participants' locations and a dark shaded area representing the approximate zone of "no copy" (aka donut hole) relative to Birmingham. Click on the image to magnify.

To represent the ability of various stations to hear each other I produced another view with color-coded lines between many (but not all) pairs of stations. Red signifies good copy and blue signifies poor or no copy. In general, stations that are at least 250 miles apart were able to copy each other, although this distance threshold seemed to be growing as time progressed beyond the 0900 CST start of the net. W4BXI reported poor copy of K9JWJ although they are 307 miles apart. I used purple to mark that path.

Hopefully, this picture will improve as sunspot cycle 24 gets going!

Museum of Radio & Technology

Visit the Museum of Radio & Technology in Huntington, WV. Their web site includes:

• A virtual tour slide show with 175 photos that include antique radios and other technology on display
• A tour of major display areas that includes captions for many of the items in photos

 A local TV news report provides a brief video tour and commentary on some of the exhibits.

Thanks to W8OI and KA4ZCO for sharing the info about this remarkable museum!

Antenna Wire Candidates

W4BXI recently shared some samples of antenna wire candidates for review and discussion. I've stripped back the tough insulation on the telco wire types to reveal the conductors and check their wire gauge. Below are photos of the samples, with captions. Click on any photo to magnify.

This first photo shows two similar drop wire types, both with copper-clad steel conductors. Telco #1 appears to be 16 ga, based on my adjustable wire stripper. Telco #2 is 19 ga, based on measurement with a telco wire gauge that has calibrated openings for 19, 22, 24, and 26 ga. The insulation is tough, but manageable with a sharp knife. The copper cladding still looked like copper after my scraping with a knife.


This photo shows two identical or very similar drop wire samples. Both have 2 pairs of 24 ga copper wire. Non-metallic (probably Kevlar) strength members are located near the outside edges of the outer jacket. The wires are located between the strength members. The outer jacket insulation is very similar to the that of the first two samples.


This last photo shows two types of single-conductor stranded wire. #4 has 7 strands of 24-ga bare copper, THHN insulation that is resistant to gasoline & oil. #5 has many strands of tinned copper wire.

Lineman's Test Set from 1940's

From WA9JNM:
You might like these pictures of my grandfather's telephone lineman box. He worked as a lineman during the 1940's in southern Indiana. The darn thing still cranks out the
voltage around 30 volts. 73's Steve

What is that single sideband?

Today's discussion on the air was good for stimulating the brain cells as we considered what a single sideband RF signal really consists of. The components of a classic AM signal example using a single, steady audio tone are familiar and are well-represented in the ARRL Handbook and numerous textbooks. In my copy of the Handbook, the chapter on Mixers, Modulators and Demodulators derives the result of mixing (multiplying) a carrier and a modulating frequency. The result is shown as:

AM signal =

sin 2f_ct + ½ m cos (2f_c - 2f_m)t - ½ m cos (2f_c + 2f_m)t

where: f_c is the carrier frequency, f_m is the modulating frequency

You recognize the first term as the carrier, the second term as the lower sideband and the third term as the upper sideband.

A simple view of single sideband would discard the carrier term and one of the sidebands. This could be implemented, for example, using a sharp filter. Examining the term that is left shows a constant sinusoid (cosine function) at a frequency above or below the original carrier frequency by an amount equal to the constant modulating frequency.

During our discussion I postulated that if I transmit a pure (single frequency) audio tone on my SSB transmitter and did not tell you where I was tuned (e.g. 3740 kHz), you could not tell, by tuning your receiver, what audio tone frequency I was transmitting. This is supported by an illustration in the Modulating Sources chapter of my copy of the Handbook. It shows a spectrum analyzer display with a single peak and an oscilloscope view of a constant amplitude RF envelope. The caption labels it as “an unmodulated carrier or single-tone SSB signal”.

Another way of saying this is: Suppose another ham tunes up with a carrier at 3738 kHz. What do you hear at 3740 kHz on your SSB receiver on LSB? You hear a 2 kHz audio tone. Now suppose I transmit a 2 kHz audio tone on my SSB transmitter on LSB on 3740 kHz. If you are listening on LSB on 3740 kHz you hear a 2 kHz tone. The effect is the same.

In the absence of more rigorous analysis, I maintain that a constant pure audio tone transmitted on SSB is equivalent to an unmodulated carrier. Of course, the real world equipment generating such a signal will add some distortion, making it not precisely identical to an unmodulated carrier. Also, a voice waveform is highly complex, with multiple varying frequencies and amplitudes.

The more rigorous treatment of SSB (example) uses math that is equivalent to the phasing method of generating SSB. It is a notch up in level of complexity compared to what is presented in the Handbook. It is also the basis of many communications systems that we take for granted today: broadband Internet access, digital TV, cell phones, etc.

Comments?

John WA5MLF

Net Manager - WA5MLF

WA5MLF (Bell Ringer #17) has assumed the duties of Net Manager for the Bell Ringers Net, effective 11/15/09. He succeeds W4BXI, whose leadership in this role is much appreciated. Any questions or suggestions about the Bell Ringers Net should now be directed to WA5MLF.

Antenna Analysis

This morning's discussion on 75 m covered the characteristics of a commercially-available doublet antenna of 130 ft total length, center-fed with 75 ft of 450-ohm open line. The following additional parameters were used in analysis performed with the 4NEC2 antenna modeling software:

• 12-gauge copper conductors
• height above ground 30 ft
• average ground characteristics

Below is a sequence of horizontal radiation patterns, computed at one frequency for each of the 9 HF bands. The patterns for 80-40 m are very similar in shape. Starting at 30 m, additional lobes appear. You can click on the figure to select and examine any slide more closely.



Using the program's 3D features, the following plot shows the complexity of the antenna's radiation pattern at 28.5 MHz. This is an overhead view, with antenna oriented from left to right. Click on the figure to enlarge.


A frequency sweep from 1.8 to 29.7 MHz was made to investigate the behavior of the antenna's impedance at the transmitter end of the feed line. The following figure shows the calculated values of resistance, reactance, impedance, and phase at increments of 0.2 MHz. Numerical values are available in the program's output file, and one set of numbers (at 7.2 MHz) are shown on the figure. Click on the figure to enlage.


It can be seen that the antenna's impedance (green curve) is fairly high (several hundred ohms) at many of the ham band frequencies, confirming the need for a tuner to achieve matching to a 50-ohm transmitter. The program includes a feature for calculating the needed matching components. This could be used to predict the ability of a given tuner to achieve a match at any given frequency.

John WA5MLF

Poll Results for Saturday Net Operations

After 7 days the web poll on this topic has closed. Thanks to all who expressed their interest in future operations of the Saturday Net by voting.

The choices, from which a voter could choose one OR more, were:

1. Use 75 m exclusively until sunspot activity enables a return to 40 m.
2. Use 75 m with optional Skype and/or phone patch connections for stations who are too distant from the core 75 m participants.
3. Use 60 m
4. Use 40 m exclusively, with the aid of relay stations. Remain on 7230 kHz while evaluating the effect of changing back to standard time on November 1.
5. Implement an RF bridge between 75 m and 40 m, enabling stations to select the band that gives them the best ability to hear and be heard.

The poll recorded 33 participants, with a total of 44 choices selected. Here is a summary of the results, which can also be viewed on the poll gadget, although you must scroll horizontally to see the numbers.

1. 75 m exclusively -- 8 votes - 24% of 33
   
2. 75 m with patching -- 3 votes - 9%
   
3. 60 m -- 1 vote - 3%
4. 40 m with relays -- 20 votes - 60%
5. 40 m - 75 m bridge -- 12 votes - 36%

Below is a pie chart showing the results based on the votes for each choice divided by the 44 total selected choices. Click on the image for a magnified view.

Thus, most responders favor the use of 40 m for the Saturday net. There is also clear indication of a desire to have the best of both worlds, combining the long range of 40 m that favors more distant members with the short range of 75 m that favors the Alabama-Georgia core of participating members. With today's technology we have several choices beyond traditional relaying to help bridge the 40 m and 75 m propagation gaps and enable all members to participate. These include a Skype-based VoIP HF Bridge and a Cross-Band Internet Bridge. These may not be available for every Saturday net, since they involve the presence and extra efforts of two members who are equipped to implement these services.

Based on our October 26 conference call, our goal in the near term is to resume a formal Saturday morning members' net with roll-call check-ins, using whatever tools are available. Weekday mornings have and should continue to hold lively, informal QSOs using two bands.

Feel free to post comments below and/or send email to me or to W4BXI.

John WA5MLF

Saturday Net Operations

In response to John's (W4BXI) call to conference about possible ways to improve our Saturday Bell Ringers Net, the following Bell Ringers met:
John (WA5MLF), John (W4BXI), Carl (W4UOA), Gary (N4OLN), Jorge (KI4SGU), Larry (KB4LWT), and Phil (KB4XX). After discussing our 40 meter propagation problems, here are some ideas we thought might make our Saturday Bell Ringers Net better:

1. Use 75 m exclusively until sunspot activity enables a return to 40 m.
2. Use 75 m with optional Skype and/or phone patch connections for stations who are too distant from the core 75 m participants.
3. Use 60 m
4. Use 40 m exclusively, with the aid of relay stations. Remain on 7230 kHz while evaluating the effect of changing back to standard time on November 1.
5. Implement an RF bridge between 75 m and 40 m, enabling stations to select the band that gives them the best ability to hear and be heard.

We invite your feedback about these choices and any others that may be relevant. Please use the poll feature at the upper right corner of this blog to capture your votes for one or several of the choices. One of our goals is to resume a formal Saturday net with check-ins by member number.

The alternatives are listed in brief form in the poll to enable better visibility of the results after your votes are entered. You'll still need to use the horizontal scroll bar to see results. (The poll feature is one that I have not previously used.)

You may also post comments directly to this blog posting for other members to read.

An updated list of Net Control Stations is posted on our web site.

Thanks for your interest and inputs!
John WA5MLF

Map of 75 m Net: 1994-1997

Following yesterday's discussion of the Net's past operation on 75 m, I looked at my logbook records and found that the Saturday morning net used 3920 kHz in the 1994 - 1997 time frame. Making note of the member stations that checked during that period I produced this Google map showing their locations to reveal the Net's viability on 75 m during the low period of the sunspot cycle. The map supports our understanding of the membership as being concentrated mainly in LA, MS, AL, GA. The highest member number among check-ins at that time was 272.

Click on the link below the map to get a larger view with more navigation features. Many of the member locations shown are only at the city level of detail, not street address. It is interesting to compare this map with the one produced earlier this year that reflects participation in the 2008-2009 time frame.

John WA5MLF


View 75 m Net 1994-1997 in a larger map

VFO Controlled Low Power Transmitter

The Rockless, a VFO Controlled Low Power Transmitter, is featured in the November 2009 issue of QST. Jimmy K9JWJ contacted the author and was directed to The Rockless QRP web site for more info. Thanks, Jimmy, for the link.

ON8PO Radio Museum

See this amazing museum of radio equipment! Thanks to W4BXI for the link!

Early telephone multiplex systems & Bell Labs

During a recent morning QSO on 3968 we talked about early frequency division multiplex technologies for carrying multiple voice channels on individual metallic circuits. Probably the earliest example was the O-carrier system deployed on open wire routes. A few photos are shown at this web site.

Information about these FDM systems is sparse on the web. The above referenced article on FDM does include a link about L-carrier systems deployed on coaxial cable.

N-carrier systems were used on twisted-pair cables starting about 1950. According to two books in my library, N-carrier was used for routes ranging from 15 to 250 miles. They used two frequency bands: 36 to 140 kHz (low group) and 164 to 268 kHz (high group) in opposite directions, on two cable pairs. At successive repeaters the signals were translated between the low group and high group to promote equalization across the entire route. Depending on the version of the system, either 12 or 24 channels were carried on 2 cable pairs.

Searching the web for more information about these carrier systems also yielded a nice article about Bell Labs. Scroll down to find entries grouped by decades.

The Bell Ringers web site also includes some good historical links on the Links page.

Latest W4BXI System Audio Drawing

Here is a drawing of John's latest audio configuration. It makes use of a second PC to 'mix' a phone line caller via phone patch into a Skype conference call that connects to the transceiver.
Click on the image above for an enlarged view. Drawing is made using the OpenOffice Draw program.

With the drawing produced, one can make copies with overlaid features, such as the one below that illustrates the main flow for a regular phone patch. A series of these can be done to form a slide presentation, if desired.

Larry's Bees

Larry KB0BH sent me some additional photos of his bee keeping and honey production. Outdoor photos were taken by N4OLN. You can view the complete set in this album on the Bell Ringers web site.

Audio Interconnections at W4BXI

Here is a draft drawing based on a sketch from W4BXI, for discussion purposes. Click on the image for a full-sized view.

VoIP HF Radio Bridge in QST

An article on this topic, written by W4UOA with some assistance from W4BXI and WA5MLF, is published in the September 2009 issue of QST, page 40. We welcome your feedback and invite you to also use the vote link for ARRL members.

Huntsville Hamfest Photos

Photos by Gary, N4OLN

These photos are also posted in the Group Photos section. Click on the thumbnail for Huntsville 2009.

Skype under threat

The VoIP technology that we like to use for HF radio interfacing to the Internet is under threat due to E-bay's sloppy acquisition of Skype. See this summary from episode 583 of Leo Laporte's show The Tech Guy. The summary includes a link to an article on the British news site The Guardian.

Pioneers award to TPARCA in 1995

Here is a bit of TPARCA history from 1995. Click on the thumbnail for a better view. Pictured receiving the award is W4AXO, Bell Ringer #187.
Also, see the 1994 group photo.

Updated W4BXI System Audio Drawing

In an earlier blog posting W4UOA provided a drawing of W4BXI' s overall audio interconnections, and WA5MLF followed with a drawing to focus on the two-way audio paths between transceiver and Internet-connected PC. Since that time W4BXI has added a second audio mixer (colored orange) to the system. An updated draft drawing by WA5MLF is shown below (click to enlarge).

Photos of key hardware elements may be seen here.

Feedback from W4BXI would be appreciated to ensure that this version of the drawing conforms with the "as built" hardware configuration.

This may aid our ongoing discussions of interfacing multiple participants to an HF radio QSO using Skype, and even the 20th century technology called a phone patch. Today's on-air discussion renewed the quest for a system configuration that can connect multiple Skype callers to the host HF transceiver without the use of external hardware mixers or a second PC to host the Skype audio conference.

Electronics Recycling in Birmingham

If you have old electronic equipment of any kind that you can't or don't want to trade or sell, consider bringing it to a recycling company, such as Technical Knock Out. They are a full-time operation located near US-31 (Independence Dr) and 28th Ave S. The annotated photo below (click to enlarge) shows the TKO location. It is suggested that you call them before going to make sure they are present. Sometimes they are away collecting discards from local businesses.

Last year I took a wide variety of items to a weekend collection that the company sponsored at Whole Foods. A couple weeks ago I took some equipment directly to the TKO warehouse where I found the employees disassembling old computer equipment and other electronic gear. They are glad to accept all kinds of radio, audio, and other equipment. Sheet metal parts are picked up by a local company that uses recycled metals to produce various forms of piping. Circuit boards are shipped to a company in another state that separates the components and materials for recycling.

Call TKO at 205-451-0180 if you have any questions.

John

N0A - Nixa, MO Field Day

N0SAP - Sap provided the following photos from the Field Day operations of the Nixa Amateur Radio Club. Click on any photo for a larger view.

Click here to view a local TV news video about the event.

Ham Radio Audio and Resources

During a discussion of online receivers this morning Bruce K4CMC told us about the blog Ham Radio Audio and Resources. If you scroll down there to the section labeled 5) Online Receiver's you'll find a list with many links. (The blog author misuses apostrophes.)

Included among them is the Lebanon, Tennessee Receiver provided by Trish Ray, K4ZE that we were remembering from an earlier QSO.

Thanks, Bruce!

Thanks to Phil KB4XX for sending an updated link the K4ZE receiver. See S-Meter Website Receivers for another listing of receivers. I suspect that these also appear in the blog listed above.

WA4PTZ Loop Analysis

Using the sketch Tim provided of his loop antenna, I entered the measurements into the 4nec2 program and got the far field patterns shown below. These are calculated with a 50-ohm generator at the feed point -- no feed line included. I expect that the field patterns would be similar with the feed line included.









160 m horizontal gain. Major lobes approximately north-south.










160 m vertical pattern. Mostly straight up, as expected.











75 m horizontal pattern. 4 lobes











75 m vertical pattern. 2 lobes; angle of radiation moving away from vertical.










40 m horizontal pattern. More lobes appearing.











40 m vertical pattern. Lobes continue to move toward lower angle.











20 m horizontal pattern. Lobes increasing rapidly and some nulls are deep.










Here's the 3D color coded version of the 20 m pattern.









20 m vertical pattern. Radiation angles continue to progress lower.











I have not looked at the patterns yet for higher bands. Below are SWR and impedance plots, without feed line included.

Creating a Horizontal Loop Antenna

Tim WA4PTZ wrote a nice paper to inspire other amateurs to discover the excellence of this simple antenna. Thanks, Tim!

W4BXI System Audio

W4BXI has gradually built a versatile interconnection of components that enable him to hear audio from two transceivers and to interface two-way audio between a transceiver and his contacts using Skype for VoIP calls via the Internet. Of course, he also incorporates the 20th century phone patch as another option.

As John's system has grown some issues have emerged about how to best interface various components while avoiding ground loops, feedback and level mismatch. Recent email exchanges with W4UOA and WA5MLF led to their developing some system diagrams to aid in discussion and identification of configuration choices. Below is a diagram from W4UOA that gives a complete picture of the capabilities assembled by W4BXI. (Click on it to enlarge.)

This shows that John can access various audio streams at the mixer as well as participate in a Skype VoIP conference via the Bridge Computer. Carl has included a list of control operator procedures describing how the main components are used. In testing today John determined that he can also initiate a Skype conference on the Rig Computer and connect those participants to his FT-1000 transceiver.

In order to focus more closely on the two-way audio paths, WA5MLF prepared the diagram shown below that does not include all components and audio mixer connections. (Click on it to enlarge.)
The audio mixer is shown with its separate left and right channel inputs from the FT-1000 and outputs to a headset. A new input to the headset's right earphone is the Skype caller's talk path (dashed line in the drawing). Phone patch #4 is used to combine the talk and listen (transceiver transmit and receive) audio while avoiding the creation of a feedback path. Phone patch #2 is used primarily for ground loop isolation, and can be replaced by a single transformer.

Comments and suggestions are welcome.

Smith Chart

Why do we need it? This is answered very well by David J. Jefferies in his article where he says:

"The SMITH chart is a graphical calculator that allows the relatively complicated mathematical calculations, which use complex algebra and numbers, to be replaced with geometrical constructs, and it allows us to see at a glance what the effects of altering the transmission line (feed) geometry will be. If used regularly, it gives the practitioner a really good feel for the behaviour of transmission lines and the wide range of impedance that a transmitter may see for situations of moderately high mismatch (VSWR)."

Many articles about the Smith chart seem to focus first on proving its mathematical validity. It is not necessary to have a rigorous understanding of its mathematical basis to get useful results from it as a tool.

A nice approach to understanding the format and applications of a Smith chart, without getting bogged down in math, is provided in the December 2007 QST, page 48. This is the first of a 3-part series that totals only 6 pages.

Another good introduction is the Smith Chart Tutorial suggested by John W4BXI. This one makes nice use of colors to illustrate the construction of the chart and includes several good simple applications. Don't be put off by the first two pages.

An impressive collection of Smith chart resources was suggested by Phil KB4XX. It includes a list of 20 tutorial articles, including the one referenced in the previous paragraph.

Several web sites provide downloadable copies of the Smith chart that you can print locally for use in working your own transmission line and matching problems. Several are included at the collection link above. My favorite sources are:

• Impedance Smith chart at Print Free Graph Paper. Once there, select Smith Chart Graph Paper on the first pull-down menu.
• Admittance Smith chart
• Combined chart (impedance and admittance scales -- color print is a must!)

Loop Antennas

This morning on 3968 we discussed loop antennas briefly. Phil referenced The Loop Skywire article from QST November 1985. Tim is pleased with the results from his loop antenna and emphasized that it need not be a full wavelength to be usable on 160 m. Larry commented about a recent club presentation about a member's 500+ ft loop antenna fed with high-quality coax and using a remote antenna tuner. The Loop Skywire article seems to emphasize keeping cost low.

N4KC published a detailed article on his loop antenna and its performance relative to other antennas installed at his location.

Another loop design that I found interesting was featured in the March 2008 QST, page 56. The author describes a 187-ft loop of #26 AWG black insulated Copperweld wire. It terminates directly on an SGC SG-237 antenna tuner mounted just below the window sill where the antenna wires enter his antenna-restricted apartment.

160 m Inverted L Antenna

The other antenna that W4BXI likes to use on 160 m is a center-fed inverted L using ladder line. The sketches below provide the configuration details. Click on the image to enlarge it.

I entered this data into the 4nec2 software as a new model for analysis. Since there is not a convenient way to depict the sloped terrain, I used a level ground terrain. The model structure is shown below as it appears in the program. White lines are the antenna elements, blue lines are the ladder line feeder, and green lines are the xyz axes.


The program's main output window is shown below. It indicates the impedance at the end of the feedline to be 131+j691 ohms (703.31 ohms @ 79.27^o) at 1.8 MHz. The SWR for a 50-ohm source is calculated to be 75.7. For comparison, the impedance at the antenna feedpoint, with feedline removed, calculates to be 38.5-j40.5 ohms (55.88 ohms @ -46.45^o).


The variation of SWR, for a 50-ohm source, is shown in the graph below.


The next set of graphs show the variation of impedance, etc. vs frequency at the end of the feedline. The values at 1.8 MHz are highlighted.


The program is able to calculate the component values for several types of networks to match the feedline impedance to a 50-ohm source. The values for each network type are shown below. Component values for a (C-L-C) high-pass T-network are highlighted in green. This gives an indication of the component ranges that may be used in an antenna tuner to achieve a match for this model approximation.


The next plot shows the far field horizontal radiation pattern for the modeled antenna. Major lobes are seen to be perpendicular to the 130 ft leg that is 60 ft above ground. It is not known how this pattern would differ for a sloped ground terrain.

The final figure shows a 2-dimensional slice of the radiation pattern, color-coded for intensity. The elevated 130 ft leg of the antenna appears as a white line emerging from the pattern. The other leg is seen as a white vertical line that connects to another white line extending to the right, close to ground. Blue lines represent the ladder line feeder.

The program enables one to rotate this pattern in all directions for other views.

ICOM IC-7600

Also from Phil:
"New ICOM IC-7600 is now shipping – first reviews are starting to come in. Wide dynamic range and many nice features - looks like another winner for ICOM. Gotta’ like that sexy hi-res display and built-in spectrum scope. Add a USB keyboard and work PSK31/RTTY. "

Solar Powered Station

Phil sends us news about this Australian Solar Powered Station.
"be sure and check out his station and photo gallery web pages – talk about a low noise floor QTH! Note the 670 amp-hour (solar-charged) battery set, his “homemade” antenna rotor/direction indicator and his “neighbors”."

Thanks, Phil.

Plot of Impedance vs Frequency

For a wider view of W4BXI's 160m antenna properties I used the 4nec2 program's feature that plots impedance vs frequency. The above plot (click to magnify) is from 1.0 to 29.7 MHz in steps of 0.2 MHz. (I started at 1 MHz to get whole numbers on the x-axis.)

The color-coded points and scales identify resistance, reactance, impedance, and phase. A nice program feature is the display of numeric values when you click on a specific point. In the above example I clicked on the 7.2 MHz point before I made the screen capture.

This plot shows the repeating peaks and nulls of impedance, etc. as the program sweeps through the chosen frequency range (with a maximum of 256 steps). The corresponding Smith chart (not included here) shows a series of 16 circles passing through the calculated impedance points plotted around the chart.

Smith Chart Views

As mentioned in the March 31 posting, I have used the Smith chart to check the calculation of impedance at the transmitter end of the 120' feedline. The starting point was the antenna's feed point impedance that is marked on the chart shown here (click on image to magnify). This calculated impedance is in the neighborhood of a dipole's impedance near resonance. The SWR circle shown is based on connecting with a feedline with characteristic impedance of 450 ohms, which is the reference point at the chart's center. The same results can be seen on a paper copy of the Smith chart when plotting with coordinates normalized to the 450-ohm characteristic impedance.


This second chart shows the location of impedance coordinates at the end of the 120' feedline, which represents 0.24 wavelength toward the generator (transmitter). The blue radial line (spoke) from the center is aligned with the 0.24 wavelength mark on the outer perimeter scale. The intersection of this line with the constant SWR circle gives the approximate impedance at the end of the feedline, as annotated in red. This value agrees, within the margin of positioning the mouse cursor, with the value calculated previously.





This final chart shows the range of impedance values (black dots along orange curve) that the program calculated when sweeping from 1.6 to 2.0 MHz. The 1.6 MHz point is identified with the green spoke line and its values are shown in the lower right-hand corner of the page.